The present invention is generally related to pulse-width-modulated (PWM) switching power supplies, and more particularly to improved control circuitry for PWM switching power supplies.
In the prior art, the three most common types of switching power supplies are the self-excited switching power supply, the frequency-modulated switching power supply and the PWM switching power supply. In the self-excited switching power supply, the frequency of operation and pulse width of the switching control signal are dependent upon the magnitude of the inductance and capacitance used in the output voltage filtering circuitry, as well as on the input voltage, output voltage, and output current. Since both the frequency of operation and pulse width of the switching control signal vary widely, the efficiency and peak current in the switching transistors of the self-excited switching power supply likewise vary widely necessitating the use of electrical circuit components that are rated to accommodate relatively high currents and high power dissipations. Furthermore, the frequency of self-excited switching power supplies may wander into the audio spectrum, thus becoming audible noise.
In frequency-modulated switching power supplies, the pulse width of the switching control signal has a predetermined length, and the number of pulses in a given time interval is varied in accordance with the output load requirements. However, the frequency of operation of frequency-modulated switching power supplies varies widely and therefore may also become audible as in the case of the self-excited switching power supply.
In PWM switching power supplies, the frequency of operation is fixed and the duty cycle of the switching control signal varies in accordance with the input voltage and the output load requirements. The frequency of operation is typically selected to be 20 kHz or higher so as to be inaudible to the human ear. For these reasons, PWM switching power supplies are generally preferrable over both the self-excited switching power supplies and frequency-modulated switching power supplies.
However, prior art PWM switching power supplies typically include an error amplifier which compares the output voltage to a reference voltage to provide an amplified error voltage for varying the duty cycle of the switching control signal. The error amplifier filters out ripple on the output voltage and also amplifies the difference between the output voltage and the reference voltage. Because of the filtering provided by the error amplifier, the response time of such PWM switching power supplies to a transient change in load current or input voltage is relatively slow compared to their frequency of operation. As the transient response is improved, the stability of such PWM switching power supplies is degraded. Also, if the frequency of operation is increased to provide faster transient response, efficiency is reduced due to increased switching losses. Thus, when using such PWM switching power supplies, tradeoffs must be made between transient response time, loop stability, switching frequency and efficiency of operation. For the foregoing reasons there is a long-felt need for an improved control circuitry for PWM switching power supplies that eliminates the need for an error amplifier and that responds rapidly to transients without degrading loop stability or efficiency.